1. Field of the Invention
This invention relates generally to a small electric motor, and more specifically to a small electric motor wherein rigidity reinforcing portions are provided for supplementing the rigidity of brushes on bent portions at which the brushes are bent to form brush bases and commutator slide portions, and auxiliary supporting portions are provided as necessary for supporting the lower sides of the bent portions so as to prevent the unwanted movement of the commutator slide portions to reduce the wear of the commutator due to friction and to extend the life of the motor.
2. Description of the Prior Art
The wear of commutator, which is one of the most important factors affecting the service life of a small electric motor, can be divided into two types; mechanical wear due to its frictional contact with brushes, and electrical wear due to sparks generated between the commutator and the brushes. The wear of the commutator becomes severe, particularly when the mechanical and electrical wear occurs simultaneously. As the commutator rotates, the brushes skip on the surface of the commutator due to the irregularities of the commutator surface or vibrations during rotation. This facilitates the generation of sparks, and therefore the wear of the commutator. Therefore, the commutator and the brushes must be kept in good contact at all times by pushing the brushes toward the commutator with an appropriate force.
In general, brushes 1 and 1' used in a small electric motor are made of strips of resilient and electrically conductive material, such as copper-beryllium alloy and brush bases 2 and 2' and commutator slides 3 and 3' are integrally formed by bending the brushes 1 and 1' at a predetermined angle, as shown in FIG. 1. Reference numerals 4 and 4' in FIG. 1 refer to terminal portions; 5 and 5' to fixing bent pieces and 6 and 6' to fixing holes. On a motor case cover 7 provided are brush supporting portions 8 and 8' and 9 and 9' for supporting the brushes 1 and 1'. Gaps between the brush supporting portions 8 and 8' and 9 and 9' form brush insert holes 10 and 10'. When mounting the brushes 1 and 1' on the motor case cover 7, the brushes 1 and 1' are inserted in the direction shown by arrows a and a' in FIG. 1, respectively. At this time, the terminal portions 4 and 4' are passed through terminal holes (not shown) provided inside the brush insert holes 10 and 10', and led out of the motor case cover 7 for external connection. Since the opposing gaps between the brush support portions 8 and 8' and 9 and 9' are determined in accordance with the thickness of the brush support portions 2 and 2', the brushes 1 and 1' are supported by the motor case cover 7 in a state where the brush bases 2 and 2' are firmly secured between the brush support portions 8 and 8' and 9 and 9'. The brushes 1 and 1' are more firmly secured to the motor case cover 7 by engaging fixing holes 6 and 6' on fixing bent pieces 5 and 5', which are formed by bending the brush bases 2 and 2', with projections 11 and 11' provided on the brush support portions 9 and 9' and staking the heads of the projections 11 and 11'. FIG. 2 is a plan view, taken from the direction shown by an arrow a in FIG. 1, illustrating the state where the brush 1 is supported by the motor case cover 7 and the commutator slide 3 is in elastic contact with the commutator. As shown in FIG. 1, the commutator slide 3 extends straightforwards in an unconstrained state, or in a state where the commutator slide 3 is not in contact with the commutator 12. When the commutator slide 3 is brought into contact with the commutator 12, however, the commutator slide 3 is bent to exert a pushing force by resiliency thereof onto the commutator 12. The pushing force can be adjusted to an optimum value by appropriately selecting the bending angle .theta. between the brush base 2 and the commutator slide 3 which is in an unconstrained state.
The rotation of the commutator, however, introduces friction between the commutator 12 and the commutator slide 3, and thereby causes a tangential force to the commutator slide 3 at the portion where it comes in contact with the commutator 12. In other words, when the commutator rotates in a direction shown by F in FIG. 2, the commutator slide 3 is brought into a state shown by a dotted line A in the figure due to a force directing toward the tip of the brush 1 (in the direction shown by T in the figure). On the other hand, when the commutator 12 rotates in a direction shown by an arrow R in the figure, the commutator slide 3 is bent more sharply and brought into a state shown by a dotted line B in the figure as a result of a force exerting in a direction to push up the commutator slide 3 (in a direction shown by C in the figure). In either case, when the degree of bending or extension of the commutator slide 3 exceeds a given limit, the commutator slide 3 is quickly restored to the original state, or in the state shown by a solid line in the figure. Thus, the commutator slide 3 repeats bending (or extension) and restoration to the original state, as described above, during the rotation of the commutator 12. When returning to its original state, the commutator slide 3 rapidly rubs the surface of the commutator 12, causing local mechanical wear due to abrasion on the surface of the commutator 12. The local wear of the commutator 12 induces sparks, leading to electrical wear. To solve this problem, it has been considered that the commutator slide in an unconstrained state is bent in advance so that the commutator slide 3 is substantially straightened when coming into contact with the commutator 12 to prevent the aforementioned repetation of bending (or extension) and restoration.
However, the brush material generally used in small electric motors is on the order of 0.08 mm in thickness, having low mechanical strength. As described in reference to FIG. 2, therefore, the rotation of the commutator 12 introduces a tension force in the direction shown by an arrow T in the figure, or a compression force in the direction shown by an arrow B in the figure to the commutator slide 3, resulting in vibration or rocking of the bent portion 13 between the brush base 2 and the commutator slide 3, or a change in the bending angle .theta. of the bent portion 13. In such a construction, therefore, the deformation of the brush as shown by a dotted line A or B in FIG. 2 has not been successfully prevented.